HP StorageWorks XP Remote Web Console Software - Manual (Page 26)

26 Continuous Access XP Journal
The journal sequence number indicates the primary data volume write sequence that the primary disk
array created for each journal group. Journal data is transferred to the secondary disk array
asynchronously with the host I/O. The secondary disk array updates the secondary data volume in the
same order as the primary data volume according to the sequence number information in the journal.
NOTE: Continuous Access XP Journal processing continues uninterrupted if the SVP reboots or fails.
Copying journals to secondary disk arrays
When a primary disk array receives a read journal command from a secondary disk array, the primary
disk array sends journal data to the secondary disk array. The secondary disk array’s initiator ports act as
host and issue special I/O operations, called remote I/Os (RIOs), to the primary disk array. The RIO
transfers journal data in FBA format using a single channel command. The primary disk array can send
several journal data using a single RIO, even if their sequence numbers are not contiguous. Therefore,
journal data is usually sent to the secondary disk array in a different order than the journal data was
created at the primary disk array. The secondary disk array ensures that journal data is applied to the
secondary data volume in the correct sequence. This method of remote I/O provides the most efficient use
of primary disk array-to-secondary disk array link resources.
NOTE: Ensure that the channel extenders are capable of supporting remote I/O. For more information,
contact your HP account support representative.
Storing journals at secondary disk arrays
A secondary disk array receives journal data transferred from a primary disk array according to the read
journal command. Journal data is stored in cache first, and then in the journal volume.
NOTE: The primary disk array does not remove target journal data from its master journal volume until it
receives the sequence numbers of the restored journal. The sequence numbers of restored journals are sent
to the primary disk array as part of the read journal command from the secondary disk array. This is true
even if the primary and secondary disk arrays are connected through a channel extender product.
Selecting and restoring journals at secondary disk arrays
The secondary disk array selects journal data to be promoted to formal data (or restored) as follows:
1. The secondary disk array uses a number as management information to distinguish the journal data
arrival from the primary disk array. If the number is 1, journal data arrived at the secondary disk array.
If the number is 0, journal data has not arrived yet. The secondary disk array determines whether or
not journal data should be settled according to this number. If journal data has not arrived yet, the
secondary disk array waits for the journal data.
2. When the top of the journal group queue indicates journal data has arrived, the secondary disk array
selects the journal data that has the lowest sequence number, and settles that journal data.
3. The secondary disk array repeats step 1 and step 2 to select and settle journal data.
Figure 6 illustrates journal data selection and settling at the secondary disk array. This diagram shows that
journal data S1 arrived at the secondary disk array because the management information indicates 1. The
secondary disk array selects journal data S1 to be settled, because S1 is the lowest sequence number.
When S1 is removed from the sequence number queue, journal data S2 becomes the top entry, but it has
Journal sequence number Sequence number assigned when the journal is obtained
Timestamp Time when journal data is obtained
Table 3 Metadata information (continued)
Type Description